Add a guide for starting and resuming workflows with `xmd`

[taras]

Story

As an Executable.md user, I want a task-oriented workflow guide, so I can start
and resume a workflow run without reconstructing the CLI contract from
specifications or pull-request history.

Common path

The guide begins with a workflow from start through resume:

xmd workflow start --id=release-1.4 --props-channel=stable flows/prepare-release.md
xmd workflow resume release-1.4

It explains what the first command creates, where the run ID appears, what is
preserved between commands, and what the resumed command continues. A reader
should understand this path before learning identity, storage, or exit-status
details.

Current gap

PR #428 shipped xmd workflow start and xmd workflow resume and documented
their use in the product README. PR #436 then made the repository README a
local-development guide and intentionally removed its product sections.

The documentation site now gives only a short comparison between xmd run and
xmd workflow. CLI help and specs/workflow-workspace-spec.md contain the
complete contract, but they are not a guide for someone trying to complete a
workflow task.

Guide

Add /docs/workflows and link it from both the Getting Started workflow card
and the documentation navigation.

After the common path, explain in this order:

1. when to use a workflow run instead of xmd run;
2. how to start with an allocated or specified run ID and properties;
3. how to resume by run ID;
4. how the workflow definition is identified;
5. how filesystem changes use the workflow Workspace;
6. what the commands report and which exit statuses they return; and
7. where local runs are stored and how XMD_WORKFLOW_RUNS selects a different
   absolute directory.

Include these runnable forms:

xmd workflow start flows/prepare-release.md
xmd workflow start --id=release-1.4 --props-channel=stable flows/prepare-release.md
xmd workflow resume release-1.4

Describe only shipped behavior. Link to specifications for detailed invariants
instead of creating a second normative contract.

Acceptance

• /docs/workflows gives a coherent start-to-resume learning path.
• A user can determine the command, required identifier, observable output,
  exit status, and preserved result for each documented operation.
• Examples match current CLI help and are covered by a site or documentation
  regression.
• Getting Started and the documentation navigation link directly to the guide.
• The repository README remains the local-development guide established by
  📝 Make the README the development guide, and run it #436.
• The page contains no roadmap or inventory of unavailable commands.

Authoritative sources

• PR 🚀 Start and resume a workflow run from the CLI (#366 PR 2) #428 — the shipped CLI lifecycle and the removed README guide.
• packages/cli/src/cli.ts — command grammar and help.
• specs/workflow-workspace-spec.md — workflow lifecycle and persistence.
• architecture.md — authority, identity, Workspace, and replay.

Relationship to #450

#450 adds a contributor entrypoint for finding repository contracts, code, and
tests. This issue adds a product guide for people running workflows.